Spray nozzle devices of the type generally contemplated are used commercially to wash wax from rotary cellular vacuum drum filters, rotary pressure filters, or belt-pan type filters. Usually the wash liquid is applied to one or more surfaces of a rotating filter cake through an arrangement of spray nozzles which are connected to a pressurized, liquid carrying distribution pipe.
For example, in rotary vacuum drum filters as are used in mineral oil dewaxing units, the filter washing agent is usually distributed onto the filter through a series of wash pipes. The latter are preferably positioned parallel to the drum axis, each wash pipe being fitted with twenty to forty nozzles. The latter are directed toward the filter surface to impinge a series of high velocity washing liquid streams thereagainst.
In the stated construction, the nozzle rows are usually arranged vertically or normally to the direction of filter rotation. The number of nozzle rows is preferably twice the number of the wash pipes, thus nozzles of adjacent even and odd numbered rows are aligned one directly behind the other. The nozzles thereby define a series of linear rows which run parallel to the rotational direction of the filter. Thus, pairs of even numbered and odd numbered nozzles define a double row.
Washing the filter in a normal manner will result in a substantially nonuniform treatment of the surface thereof. The wash surface will usually be characterized by a series of parallel striped areas which have been sufficiently treated, in contrast to the striped areas adjacent thereto which have not been contacted by adequate washing liquid.
Among the forms of nozzles which are typically applied to the present apparatus are the well jet, full jet type, or even special nozzles can be used. Although good spraying patterns can be achieved with different types of nozzles, these patterns provide scant information regarding liquid distribution on the wash surface.
If the amount of washing agent which is emitted by a nozzle, is integrated over the nozzle's sprayed circular segment, to determine the distribution of the liquid on the moving cake, it will be noted that the washed surface is unevenly sprayed. Further, the intensity of washing will vary within the width of a jet cone.
In addition, the nozzle's spraying angle and the distribution of liquid in the spraying pattern on the filter, depend on the fluid pressure source as well as on the quantity of the fluid used.
In the instance of a rotary drum filter such as that illustrated in FIG. 2, for "n" number of nozzle rows and the nozzle aligned arrangement as described above, the filter surface area which is sprayed by the individual nozzles, covers the filter surface n/2 times. Therefore, within the nozzle spraying pattern, these quantities, deviating from the mean value of the minimum and maximum quantities, are additive by a factor of the number of "n" lines of nozzles or n/2.
Consequently, when passing across the washing zone, some segments of the filter cake are contacted repeatedly with large amounts of washing medium. Other segments of the filter (on the other hand) are sprayed repeatedly with a paucity of wash liquid.
The uneven distribution of the washing fluid caused by the usual arrangement of nozzles results in surplus wash liquid running unused down some areas of the filter drum. In addition, channeling occurs, a phenomena wherein wash liquid penetrates areas of the filter cake that offer less resistance to penetration.
Other areas of the filter cake receive smaller quantities of washing medium due to their resistance to liquid penetration. In summary, a considerable quantity of the wash liquid is uselessly wasted due to the spotty overdosage of some areas and the insufficient washing of other filter areas.
The prior art teaches that to insure all areas of the filter cake being sufficiently washed when the usual nozzle arrangement is used, it is necessary to furnish an excessive amount of wash liquid.
Also, washing of the filter in the normal manner will result in non-uniform treatment. The filter surface as herein noted will be characterized by a series of striped areas which have been sufficiently treated, in contrast to striped areas adjacent thereto which have not been sufficiently treated.
It is therefore an object of the invention to provide a filter washing apparatus including a plurality of nozzles that will effectively and efficiently treat the filter surface. A further object of the invention is to provide a filter washing nozzle assembly which minimizes the amount of cleaning, or wash liquid which is required to adequately treat a filter surface. A still further object is to provide a nozzle array or a filter washing member, which is capable of uniformly treating a moving filter surface.
The above noted problem endemic to filter cake treating or washing, is overcome by the instant invention through a unique arrangement of the liquid carrying nozzles. The latter are positioned in such manner that the respective nozzles in any double row thereof, are positioned in staggered arrangement relative to succeeding double rows. Further, such arrangement is disposed transversely to the direction of movement or the rotation of the filter cake.
Within any double row of nozzles, the distances between individual spray nozzles is constant. The staggered nozzle arrangement with the double row, insures that wash liquid is sprayed uniformly across the width, and onto all areas of the filter cake. This nozzle array is found to utilize a minimum quantity of wash liquid while improving the washing efficiency of the entire unit.